The invention relates to an interferometer and, more particularly, to an interferometer constructed using integrated-optics technology. The interferometer includes a baseplate with waveguides integrated therein. A portion of the integrated waveguides, i.e., the entrance portion, is passed to the edge of the baseplate and is there connected in a light-conducting manner to an entrance optical fiber supplying primary light from a laser light source. A further portion of 10 the integrated waveguides, i.e., the exit portion, is likewise passed to the edge of the baseplate and is there connected in a light-conducting manner to an exit optical fiber conducting interference light away to a detecting device. The interferometer further has an integrated light-splitting device, disposed downstream of the entrance portion of the integrated waveguide, to split up the injected primary light onto a reference arm and onto a measuring arm of waveguides. That portion of the waveguide which forms the reference arm is integrated over its entire length in the baseplate. An integrated light coupler combines the light from the reference arm and that from the measuring arm and merges into the exit portion.
Such an interferometer is disclosed, for example, in an article in the German periodical: tm-Technisches Messen 58 (1991), No. 4, pages 152 to 157, R. Fuest, "Integrated-optics Michelson interferometer with quadrature demodulation in glass for the measurement of displacement paths". What is designated in this case, for reasons of clarity, as a baseplate is in most cases designated as a "substrate" in the scientific literature. As regards the physical form of the base carrying the waveguides, this does not indicate any concept, which is why the term "baseplate" is preferred here.
In the interferometer known from the above cited literature reference, only the reference arm is integrated in the baseplate, whereas the measuring arm is integrated in the baseplate only to the extent of a proportion which is small and above all metrologically passive. In specific terms, the integrated part of the measuring arm is passed to the edge of the baseplate, where it is injected via a small lens into a measuring section which is situated outside the baseplate and at the end of which a reflector is disposed. The displacement path is to be determined by interferometry. Accordingly, this literature reference describes a free beam interferometer of the Michelson type of integrated optics miniature construction. This literature reference also describes integrated-optics structures of waveguides for the generation of phase-shifted interference signals, with which not only does the direction of movement, i.e., movement towards or away, become discernible, but also a higher degree of measurement resolution becomes attainable. One of the waveguide structures described here is the so-called 3.times.3 directional coupler, with which three interference signals with a phase shift of preferably 120.degree. in each instance can be generated. This literature reference does not discuss the question of a specific metrological application of the miniaturized free-beam interferometer. Furthermore, that is not the intention of this article. Reference is only made to measurements of displacement of the component carrying the reflector at the end of the measuring arm. In addition to the miniature construction, emphasis is placed, inter alia, on the disturbance protection in relation to electromagnetic fields by way of advantage.
Another article in the same periodical tm-Technisches Messen 58 (1991), No. 4, pages 146 to 151, by G. Ulbers, "Integrated-optics silicon-based sensors for path, force and refractive index measurement", in which miniaturized free-beam interferometers of the Michelson type are also described, mentions various metrological practical applications of such sensors. In one of the cases of application which are presented, the extension is measured by interferometry on a tension specimen which is clamped into a material testing machine and is increasingly subjected to tension and extended. In specific terms, spherical reflectors projecting transversely from the specimen are fitted to the two ends of the tension specimen. The reflectors reflect an interferometer measuring beam injected parallel to the specimen axis from the end surfaces again parallel to the end-surface interferometers. The interferometers themselves are not fitted to the tension specimen, but so as to be stationary on the machine frame of the material testing machine. It is emphasized that the interferometric extension measurement brings the advantage of a non-contact measurement and a high resolution, as compared with conventional extension measuring methods. A disadvantage of the known type of interferometric extension measurement is the very awkward application, which is very bulky as compared with the electrical-resistance extension measurement using wire strain gauges.
A further article in the periodical Feinwerk-technik & Me.beta.technik 97 (1989), No. 10, pages 415 to 421, by K. Gro.beta.kopf, "Use of glass for integrated optics", shows diagrammatically, inter alia, an interferometer according to the Mach-Zehnder type, which is fully integrated, i.e. integrated into a baseplate both with the measuring arm and also with the reference arm, but in which both rectilinearly-designed interferometer arms extend parallel side by side in the baseplate. If the optical conditions are altered by a quantity to be measured in one of the parallel interferometer arms in comparison with the reference arm, then the interference pattern in the exit channel is altered. This permits the derivation, following optoelectronic evaluation, of a measurement value, for example, for substance concentration or magnetic fields in the surrounding medium. No thought is given to an application of this interferometer for extension measurements, nor is such application possible.
There is therefore needed an interferometer which can be used in a manner which is just as simple and space-saving as a conventional electrical-resistance wire strain gauge.
According to the present invention, this need is met on a two-fold basis, namely on the one hand by a double-beam interferometer according to Michelson or according to Mach-Zehnder and on the other hand by a single-beam interferometer according to Fabry-Perot. The double-beam interferometer includes a baseplate with waveguides integrated therein. A portion of the integrated waveguides, i.e., the entrance portion, is passed to the edge of the baseplate and is there connected in a light-conducting manner to an entrance optical fiber supplying primary light from a laser light source. A further portion of the integrated waveguides, i.e., the exit portion, is likewise passed to the edge of the baseplate and is there connected in a light-conducting manner to an exit optical fiber conducting interference light away to a detecting device. The interferometer further has an integrated light-splitting device, disposed downstream of the entrance portion of the integrated waveguide, to split up the injected primary light onto a reference arm and onto a measuring arm of waveguides. That portion of the waveguide which forms the reference arm is integrated over its entire length in the baseplate. An integrated light coupler combines the light from the reference arm and that from the measuring arm and merges into the exit portion. Both that portion of the waveguide which forms the reference arm and also that portion of the waveguide which forms the measuring arm are in each instance integrated over their entire length in the baseplate. At least that portion of the waveguide which forms the measuring arm is disposed in a plurality of loops with mutually parallel oriented straight active partial sections and intermediate deflecting sections, in which a coupling-free mutual minimum spacing of all straight active partial sections and deflecting sections of the integrated waveguide forming the measuring arm is observed at all locations. The waveguide forming the reference arm is designed in such a manner and/or is disposed in the baseplate in such a manner that upon extension of the baseplate in the direction of the active partial sections of the measuring arm, the reference arm behaves in an extension-metrologically neutral manner.
The interferometer of the Fabry-Perot type is constructed using integrated optics technology. It comprises a baseplate with waveguides integrated therein. A portion of the integrated waveguides, i.e., the entrance portion, extends to the edge of the baseplate and is there connected in a light-conducting manner to an entrance optical fiber supplying primary light from a laser light source and a further portion of the integrated waveguides, i.e., the exit portion, likewise extends to the edge of the baseplate and is there connected in a light-conducting manner to an exit optical fiber conducting interference light away to a detecting device. The interferometer further has a Y-shaped combining element combining the entrance portion and the exit portion into a common waveguide. Downstream of the common waveguide of the combining element, there is optically disposed a resonator. The resonator is formed from a partially transmitting mirror, which is integrated into the baseplate and which stands orthogonally to the waveguide. An integrated resonator waveguide and an end mirror are disposed at the other end of the resonator waveguide. The resonator waveguide is disposed in a plurality of loops with mutually parallel oriented straight active partial sections and intermediate deflecting sections, in which a coupling-free mutual minimum spacing of all straight active partial sections and deflecting sections of the integrated waveguide forming the resonator is observed at all locations.
By virtue of the full integration of both interferometer arms into the baseplate of the integrated-optics double-beam interferometer and of the extension-metrologically neutral design or arrangement of the reference arm therein on the one hand and, by virtue of the multiple loop-type course at least of the measuring arm in the baseplate, the interferometer according to the present invention is not only made quite considerably smaller and simplified as compared with the known free-beam design, but also its application to components to be stressed becomes, in principle, just as simple as that of an electrical resistance wire strain gauge. It is amazing that the integrated waveguides can be integrated into the base plate with relatively tight curvature and also with a mutually crossing course, with low loss. The double-beam interferometer can, as stated, be designed as a Michelson interferometer or as a Mach-Zehnder interferometer. The Fabry-Perot extension interferometer functions in a similar manner. However, with respect to its advantages it behaves in principle on an equivalent basis.
In order to achieve an automatic compensation of the extension due to temperature, in the case of the double-beam interferometer, the reference arm is also expediently wound in the manner of loops and is designed to be of the same length as the measuring arm and in this case is of course disposed in an extension-metrologically neutral manner. A comparable design for a single-beam interferometer is also disclosed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.